Inhaled nitric oxide (INO) and supplemental oxygen are used clinically in the treatment of pulmonary diseases associated with respiratory failure in infants. In preterm and term human infants, oxygen and/or nitric oxide therapy may contribute to lung injury. The long-term goal of this application is to develop rational strategies for the prevention/treatment of lung pathologies in infants exposed to hyperoxia and INO. This application comprises 4 Specific Aims: (1) To characterize the specific mechanisms by which hyperoxia and INO, alone or in combination, modulate hepatic and pulmonary cytochrome P450 (CYP) enzyme expression, in relation to acute lung injury. Newborn rats will be exposed to hyperoxia (geater than 95 percent oxygen), INO, a mixture of hyperoxia and INO, or maintained in room air for selected time points for up to 7 days. Message levels [(Northern hybridization, reverse transcriptase polymerase chain reaction (RT-PCR), in situ hybridization), apoprotein contents (Western blotting and immunohistochemistry), and enzyme activities (fluorimetry) of specific CYP isoforms, i.e. CY`PlAl/lA2 and CYP2EI will be determined in the liver and lung. (2) To test the hypothesis that endogenous NO contributes to the downregulation of specific CYP enzymes by hyperoxia, and to the mechanism(s) of lung injury by hyperoxia. Mice lacking the gene for NOS2 or NOS3 will be used to specifically test the hypothesis that endogenous NO contributes to hyperoxic lung injury. (3) To determine the possible role of the aryl hydrocarbon receptor (AHR) in the differential susceptibilities of newborn rodents exposed to hyperoxia or hyperoxia + INO. AHR null mice will be used to determine the possible role of the AHR in acute lung injury. (4) To test the hypothesis that neonatal imprinting of CYP enzymes caused by exposure to hyperoxia, [NO, or hyperoxia + INO contributes to abnormal lung maturation in adult rats. The results will provide critical information about the acute effects of INO and hyperoxia on CYP expression, and the contribution of neonatal imprinting of these enzymes in abnormal lung maturation occurring through adult life. The studies will also help identify appropriate supportive therapies to minimize any adverse effects arising out of this clinically important therapy.